منبع پایان نامه درباره literature review
220.127.116.11.2 Conformation in solution and conformational transition
The properties of κ-carrageenan and ι-carrageenan were the subject of numerous studies (Morris, 1998; Piculell, 2006; Vreeman, Snoeren & Payens, 1980). They depend on both temperature and ionic environment (presence of salts, concentration). In solution (hot), carrageenans behave as random coil whose degree of expansion depends on the degree of sulfation and the ionic environment due to its polyelectrolyte character. On cooling, they adopt a helical conformation. The temperature of helix-coil transition which depends on the nature of the cation (K+, Ca++ …) and ionic strength can be observed by polarimetry measurements (Rees, 1969), conductivity (Rochas & Rinaudo, 1980; Rochas & Rinaudo, 1982), light scattering, differential thermal analysis (Lahaye, Yaphe & Rochas, 1985; Morris, Rees & Robinson, 1980; Rochas & Rinaudo, 1982) (Figure 2.18).
Figure 2. 21: Percentage of order of κ-carrageenan solution by polarimetry (0) and conductivity measurements (y)(Rochas & Landry, 1987)
The ionic state of carrageenan and ionic environment which it is determine its conformational properties. Rochas (1982) revealed by polarimetry and by conductivity measurements that the different cations studied can be classified into three categories according to their effectiveness in promoting and stabilizing the formation of helix:
Monovalent cations nonspecific: Li+ Na+ N(CH3)4+
Divalent cations: Co2+ Zn2+ Mg2+ Sr2+ Ca2+ Ba2+
Specific monovalent cations: NH4+ K+ Cs+ Rb+
Specific monovalent cations are very effective in promoting formation the helices. For non-specific monovalent cations and divalent cations, the effectiveness of stabilization of the helices varies little. For example, it is found that the “small” ions such as Na+ and Li+ or very large such as N(CH3)4+ does not favor the establishment of an ordered conformation (figure 2.19). They are only able to form ionic bonds with the sulphate groups, but does not form electrostatic bonds with oxygen atoms present in the residue anhydrogalactose (Rochas & Rinaudo, 1982).
Figure 2. 22: Change in transition temperature Tm at cooling κ-carrageenan based on the total concentration of CT different monovalent cations (1) Rb+, (2) Cs+, (3) K+ ,(4) NH4+, (7) N(CH3)4+ (8) Na+, (9) Li+ and divalent cations (5-6) Ba2+, Ca2+, Sr2+, Mg2+, Zn2+, Co2+
Rochas (1982) has shown for κ-carrageenan in pure ionic form, the reverse of Tm (helix/coil transition temperature) varies linearly with the logarithm of the total concentration of cation. The total ionic concentration of the medium is defined by the following equation:
CT = CS + γ CP Equation 2.1
With Cs: concentration of added salt
CT: the total concentration of salt
CP: concentration cons-ions provided by the polymer
γ: Activity coefficient taking into account the electrostatic interactions between cations and the polymer (0.55 in the case of κ-carrageenan as K+)(Lafargue, Lourdin & Doublier, 2007).
The increase of Tm with CT indicates that the ordered conformation is stabilized with increasing ionic strength. The charges on the polymer are shielded and repulsions decrease, which contributes to lowering the electrostatic free energy of the helix. The linear variation CT= f(Tm-1) observed for different cations is generally observed for the conformational transition of polyelectrolytes such as glucose. The theory of condensation of cation provides that the slope of the line is proportional to the enthalpy of transition taking into account variations in charge density during the transition. A good correlation was observed with the enthalpies of transition measured by calorimetry (Rochas & Rinaudo, 1982).
This type of representation, similar to a phase diagram, provides a simple way to calculate the temperatures of transitions from the polymer concentration and ionic strength of the medium. It also implies that in the absence of added salt, Tm depends essentially on the polymer concentration. Rochas (1980) relies on the construction of a phase diagram for κ-carrageenan under K+ in aqueous solution in the presence or absence of KCl from which it has identified several areas where carrageenan is found in different forms (Figure 2.20):
Domain I: κ-carrageenan has a disordered conformation in the form of random coil as the temperature is above Tm
Domain II: κ-carrageenan has an ordered conformation as a dimer of helix as the temperature is below Tm but CT is less than C*, the threshold of gelation.
Domain III: κ-carrageenan has an ordered conformation in the form of aggregated helices and form a gel.
Figure 2. 23: Phase diagram of κ-carrageenan representing the variation of transition temperature on cooling and heating according to the total concentration of potassium (Rochas, 1982; Rochas & Rinaudo, 1980).
Below the concentration C*= 7×10-3 eq/l or Tm= 20°C, the transition temperature of helix-coil and coil-helix are combined. For concentrations above C*, there is a hysteresis between the transition temperature on cooling (transition coil/helix) and heating (transition helix/coil).
This hysteresis increases with the concentration of ionic environment. The existence of this hysteresis is a reflection of the process of aggregation of double helices of carrageenan. Aggregates of double helices are more thermally stable and therefore the transition temperature of helix/coil is above the transition coil/helix. The calorimetric measurements can highlight the process of disintegration and return to the disordered state (Rochas & Rinaudo, 1982).
18.104.22.168 Gelation of κ-carrageenan
The gelation of κ-carrageenan was the subject of many studies. We limit ourselves to consider the essential mechanisms and properties. For more details, you can refer to literature reviews(Piculell, 2006; Rees, 1969). Early studies of Rees (1969) based on experiments of X-ray diffraction on films of ι-carrageenan had resulted in a first model of carrageenan gel in one step (coil/double helices). Morris et al (1980) have subsequently proposed for κ-carrageenan mechanism involving two distinct steps and to take into account the specific ion (Figure 2.21).
Figure 2. 24: κ -Carrageenan gelation model, cation to promote gelation. (Morris et al., 1980)
The first step is the conformational transition from coils to double helices and the second stage results in the aggregation of these double helices. Gelation come from the aggregation of double helices by ionic junctions K+ in the presence of KCl. The formation of three-dimensional network requires, according to this model, the presence of “bends” at the primary structure, interrupting the regularity of structure and giving each channel can participate in several double helices with other channels. This role of “bend” would be played by replacing some galactose unit residues anhydrogalactose.
In turn, Smidsrød et al. (1980) proposed a model of gelation from specific branch simple propellers also involving the K+ ions. Rochas (1982) then showed that κ-carrageenan could adopt a conformation ordered without gelling. This author shows that the formation of helices does not necessarily form a gel. A critical concentration of ions C at a given temperature is necessary for the dimer of helices associate into aggregates that are then “bridged” by weak bonds. This assumption excludes the need to directly involve K + ions in the junctions and the presence of “bends” the strings (Lafargue, Lourdin & Doublier, 2007).
Furthermore, it has been shown that the presence of certain anions, iodide or thiocyanate for example, gelation cannot occur. This is attributed to a high charge density of double helices, thus limiting their aggregation (Borgström, Piculell, Viebke & Talmon, 1996; Viebke, Piculell & Nilsson, 1994; Zhang & Furó, 1993). There is however a gel by dialysis against a specific salt such as KCl. Thus, if the nature of the helix dimer is unclear, it is commonly accepted that gelation of κ-carrageenan is primarily the result of a “pile” of helix dimers involving a large number of them to form “bundles”, as has been suggested on the basis of results obtained by Diffusion RX small angle (Turquois, Rochas, Taravel, Doublier & Axelos, 1995).
22.214.171.124 Thermoreversibility of gels and rheological properties
By uniaxial compression measurements, Rochas et al. (1990) found that the mechanical properties of gels of κ-carrageenan depending on the molar mass. Below a critical molar mass (3×104 g/mol), the gel cannot be formed. Beyond this value, the Young’s modulus increases with molar mass up to 2 × 105 g/mole then stabilize. The same authors have shown that there is also a critical concentration of gelation, it is close to the critical entanglement concentration (C*) solutions of polymers. Beyond this concentration, the Young’s modulus follows a change in the type E = k Cν with close ν to 2 (Rochas, Rinaudo & Landry, 1990).
The harmonic tests used to characterize the viscoelasticity of gels and to study their thermoreversibility. Note that it is necessary to ensure that measures are undertaken in the absence of landslides. For κ-carrageenan, these effects are more likely to occur by placing high in K+ or high]]>